Field of the Invention
The invention relates to a method for establishing an embedded optical communication channel in an optical wavelength division multiplex (WDM) transmission system as well as to a network device configured to implement this method and to a respective optical WDM transmission system.
Description of Related Art
An optical WDM transmission system generally comprises a plurality of first channel transceivers at a first end of a bidirectional optical WDM transmission path, a plurality of second channel transceivers at a second end of the optical WDM transmission path, each first channel transceiver being configured to create and transmit a first optical channel signal, in a respective downstream channel, to the second end of the optical WDM transmission path and to receive a second optical channel signal created by a second channel transceiver and each second channel transceiver being configured to create and transmit a second optical channel signal, in a respective upstream channel, to the first end of the optical WDM transmission path and to receive a first optical channel signal created by a first channel transceiver.
In addition to the optical transmission links which are established between pairs of dedicated first and second channel transceivers (of course, broadcasting links between a first and a plurality of second channel transceivers or vice versa are possible, too) and which are provided for transmitting the (usually high bit rate) wanted optical signals or line data signals, an optical WDM transmission system may require to transmit additional control information, for example information that is necessary to control the wavelength and/or optical power of the optical sources, such as tunable lasers.
Such control information may be transmitted by providing separate control channels using separate transmission paths or by means of so-called embedded communication channels (ECC), which can be operated in-band or out-of-band of the optical transmission links for the wanted optical signals. That is, the information transported in the ECC uses the same optical transmission path as the wanted optical signals.
It is known to modulate the amplitude of a wanted (high bit rate) signal in the electrical domain according to a low-frequency or low bit rate control signal before converting this combined signal into an optical signal that includes an ECC for transporting the control information included in the control signal in addition to the optical channel signal. That is, the modulation depth of the optical channel signal is varied according to the low-frequency control signal (see e.g. U.S. Pat. No. 7,421,199 B2).
If one or more ECCs are used in order to control the wavelength of a plurality of tunable optical sources, for example tunable optical lasers that are provided within respective channel transceivers comprised by an optical line terminal (OLT), a centralized wavelength control may be implemented using a separate (stand-alone) device. In the European patent application EP 2 773 056 A1, the applicant disclosed a method for tuning a tunable optical transmitter in an optical WDM transmission system using a central or stand-alone network device, which is configured to extract, from a downstream optical WDM signal, control information that is included in one or more downstream ECC channels realized by amplitude-modulating a respective optical downstream channel signal. The upstream ECC channels are realized by modulating the upstream optical WDM signal according to one or more low-frequency sub-carriers (pilot tones), which are modulated according to a respective (even lower-frequency) control signal including the control information to be transmitted in the upstream direction.
However, this method for realizing an upstream or downstream ECC requires the presence of an upstream optical WDM signal, i.e. at least one upstream channel signal. This, however, cannot be guaranteed in all circumstances.
It is thus an object of the present invention to provide a method for establishing an embedded communication channel (ECC) in an optical WDM transmission system as well as a network device configured to implement this method and a respective optical WDM transmission system, wherein no optical WDM signal is required in order to establish the ECC and wherein the ECC between the network device and one or more channel transceivers at a respective end of the WDM transmission path can be implemented in an easy and cost-efficient way.